Processing an audio input signal to produce a processed audio output signal

ABSTRACT

An audio input signal is processed to produce a processed audio output signal. An audio input signal is received as an original signal. The audio input signal is dynamically filtered to produce a first stage signal consisting of a selected frequency band of the input signal. Gain applied to the first stage signal is dynamically controlled in response to a control signal to produce a second stage signal. The control signal is derived from the first stage signal. Processing the original signal in combination with the second stage signal to produce a processed audio output signal. Processing the original signal in combination with the second stage signal and the first stage signal to produce a processed audio output signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to United Kingdom Patent ApplicationNo. 05 26 143.3 filed 22 Dec. 2005, the entire disclosure of which isincorporated herein by reference in its entirety as if fully set forthherein.

FIELD

The present invention relates to a method of processing an audio inputsignal to produce a processed audio output signal and to an audio signalprocessing apparatus for processing an audio input signal to produce aprocessed output signal. The invention also relates to a computerreadable medium having computer readable instruction executable bycomputer such that, when executing these instructions a computer willprocess an audio input signal to produce a processed audio outputsignal.

BACKGROUND

Audio signals may be derived from a variety of sources, and may besupplied to an audio processing environment for processing. An audioprocessing environment may include a mixing desk having processingfunctionality and parameters that are controllable by an operator.

SUMMARY

According to an aspect of the present invention, there is provided amethod of processing an audio input signal to produce a processed audiooutput signal, comprising the steps of: receiving an audio input signalas an original signal, dynamically filtering said audio input signal toproduce a first stage signal consisting of a selected frequency band ofsaid audio input signal; dynamically controlling gain applied to saidfirst stage signal in response to a control signal to produce a secondstage signal; deriving said control signal from said first stage signal,and processing said original signal in combination with said secondstage signal to produce said processed audio output signal.

According to a further aspect of the present invention, said originalsignal is processed in combination with said second stage signal andsaid first stage signal to produce said processed output signal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an environment in which audio signals are processed;

FIG. 2 illustrates the digital mixing desk identified in FIG. 1;

FIG. 3 illustrates operations performed in the mixing desk of FIG. 2;

FIG. 4 details the channel processing system identified in FIG. 3;

FIG. 5 illustrates the first embodiment of the present invention;

FIG. 6 illustrates a second embodiment of the present invention;

FIG. 7 illustrates a preferred approach for deploying the functionalityof the circuit illustrated in FIG. 6;

FIG. 8 shows a third embodiment of the present invention; and

FIG. 9 shows an alternative arrangement of FIG. 8.

WRITTEN DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION

An environment in which audio signals are processed is illustrated inFIG. 1. Audio signals may be derived from many sources and three suchsources are identified in FIG. 1, by way of example only. The firstaudio source may be generated by an interview 101, possibly to berecorded or, alternatively, to be broadcast immediately.

The second audio source is identified as theatrical 102, this being inthe form of a television programme being recorded or a cinematographicfilm being produced. Thirdly, recording section 103 is identified as athird source of audio material which, on this occasion, will result inthe release of audio material but may require substantial amounts ofprocessing and mixing before the final results are produced.

In addition, in FIG. 1, typical output recipients are identified. At 104a broadcasting environment is shown, that may take the form of an audiobroadcast, a television broadcast or an internet broadcast etc. For thisenvironment, mixing and processing operations must be conducted inreal-time, given that the material is being sent to air immediately.

Similarly, an environment for film or video recording is illustrated at105. In this environment, it is normal practice for the audio assets andvideo assets to be processed separately before being combined in thefinal edit. The particular technique to be deployed will also bedependent on the effect to be achieved and the overall budget madeavailable for post production activities.

Thirdly, an audio recording environment 106 is shown which, in thisexample, represents the process of taking a mixed and processed stereosource, recording it to a master medium and then subsequentlyduplicating the recording for distribution.

In the broadcasting and video recording environment 104 and 105, it isappreciated that images are mixed and processed and the processing ofvideo material is illustrated at 107. For all of the environments shown,it is necessary to receive audio material, mix this material andsubsequently process this material. An environment for achieving thismanipulation of the audio assets is illustrated at 107. Audio processingenvironment 107 is also detailed in FIG. 2.

At the heart of the audio processing environment, there is provided adigital mixing desk 201. For the purposes of this illustration, it isassumed that all of the required mixing and processing functionality iscontained within the mixing desk 201; although it is appreciated that inalternative environments additional equipment for particularapplications may be interfaced to the mixing desk 201.

The mixing desk 201 receives audio sources which, in this example, areillustrated as microphones 202. However, it should be appreciated thatany audio source may be processed in this way and the number of audiochannels being processed may vary significantly. Furthermore, it is alsoappreciated that live data may be mixed and processed in combinationwith recorded data.

Again for the purposes of illustration, the mixing desk 201 provides astereo output 203 to a recording and broadcast system 204. Theenvironment is also provided with loud speakers 205L and 205R tofacilitate monitoring while the mixing operation takes place. Themonitors receive an output signal from a power amplifier 206, that inturn receives a stereo output from the mixing desk 201.

An operator is also provided with stereo headphones 207 and it should beappreciated that an operator may receive the same output 203 for boththe monitors 205 and for the headphones 207 or, alternatively, othersignals may be supplied to the headphone channel so as to facilitate themixing procedures.

The mixing desk itself includes a plurality of sliders 208 for adjustingthe levels of the individual channels. In this example, the desk iscapable of receiving eight audio inputs and for that eight input sliders208 are provided. However, it should be appreciated that this is forillustrative purposes only and substantially more audio channels may beprovided on typical professional mixing equipment.

A bank of the visible displays 209 ensures that each individual channelmay be monitored. Similarly, meters 210 are provided for monitoring thestereo mix.

The user interface also includes many other rotary controls tofacilitate adjustment of individual channels and the mix as a whole.Traditionally, this includes controls for adjusting filter parametersand, traditionally, having these parameters set to particular conditionswould result in a frequency response for the channel being setpermanently, unless further modifications are made. Thus, traditionally,an operator would have direct control of the parameters themselves andthe operator would be responsible for controlling the way in which theprocessing functionality modifies and mixes the incoming signals so asto produce the final result.

In this example, the mixing desk is also provided with a computer-likeinterface 211, that may be particularly suited to working withmulti-channel outputs as described in the present applicant's Britishpatent 2277239 and British patent 2294854.

Operations performed in mixing desk 201 are shown schematically in FIG.3. Each input channel has a dedicated channel processing system which,in this example, eight are identified as 301 to 308. Each channelprocessing system (301-308) allows input signals to be processedindependently and the channel itself is provided with substantialprocessing capability.

After processing each input channel individually, a mixing operation isperformed at mixing sub-system 309 in order to produce mixed signals,each being made up of signals received from a plurality of the inputchannels. In this example, a stereo output is produced such that a mixedsignal is provided to a left channel processor 310 and to a rightchannel processor 311. However, it should be appreciated that, in someenvironments, more output channels are required and the stereo system ofFIG. 3 is shown for illustrative purposes only.

Thus, the collection of input signals are received which, inprofessional equipment, could amount to substantially more than theeight (8) of FIG. 3. Being separate signals, they can be combined bymixing in order to produce output signals; of which 310 and 311represent a stereo mix. It should also be appreciated that other outputsignals are produced, such as those provided to the internal monitors205 and to the stereo headphones 207.

Channel processing system 301 is detailed in FIG. 4. An analogue inputsignal is supplied to an analogue to digital converter 401, that in turnprovides a digital signal to a digital signal processing environment402. In this embodiment, the analogue to digital converter 401 producesa 24 bit digital signal, submitted on a bus 403 to the digital signalprocessing environment 402.

Within the digital signal processing environment 402 itself, highdefinition floating point manipulations are performed, typically at 36bits or at 40 bits internally within the processors. Furthermore, aninterface circuit 404 ensures that the output from the digital signalprocessing environment 402 is reconverted into a form of representationcompatible with mixing sub-system 309.

In terms of the hardware realisation of the system shown in FIG. 4, andas would be known to those skilled in the art, an engineering assessmentis made in terms of the degree of processing required in order toprovide the desired level of processing capability. Presently, systemsof the type shown in FIG. 4 are implemented on physical processingboards containing typically 12 digital signal processing (DSP) chips,such as those produce by Sharp Inc of Japan. In preferred embodiments,to be described with reference to FIGS. 5, 6 and 8, a filteringoperation is performed and a gain control operation is performed. Inthis illustrative embodiment, a whole DSP processing chip would beallocated for each of these operations for each individual channel.

However, it should be appreciated that the functionality could also beachieved by means of a more general purpose processing environmentalthough, eventually, a limitation would be reached in terms of thenumber of channels that could be processed, due to the availability ofprocessing capability. However, in an alternative embodiment, it wouldbe possible for a general purpose processing system to be programmed toachieve some of the inventive results. Furthermore, it would also bepossible for standard type media containing computer readableinstructions to be distributed and subsequently installed by computerusers so as to achieve some of the claimed methods.

The present embodiments are all concerned with the processing of anaudio input signal to produce a processed audio output signal. An inputsignal is dynamically filtered to produce a first stage signalconsisting of the selected frequency band of the input signal.Thereafter, gain is applied to the first stage signal that isdynamically controlled in response to a control signal so as to producea second stage signal. The control signal itself is derived from thefirst stage signal. Thereafter the second stage signal is processed incombination with the original audio input signal to produce theprocessed output signal.

A first implementation of the above approach is illustrated in FIG. 5.As previously stated, DSP chips within the digital signal processingsystem are allocated in order to provide the functionality of the filter501 and the gain control 502. A modest side chain processing circuit 503is also provided, along with a summation circuit 504.

In the following, reference is made to an “input” signal and to an“output” signal. These refer to the inputs and outputs respectively ofthe circuits shown in FIGS. 5, 6 and 8 and not to the overall input andoutput signals of type shown in FIG. 2.

Thus, in the circuit of FIG. 5, an audio input signal 505 is received asan original signal that is supplied to the summation circuit 504 and tothe filter 501. The filter 501 allows a particular frequency band of thereceived input signal to be selected. In the preferred embodiment, thisis achieved by the provision of two high pass filters, for attenuatingfrequency signals below the selected band, and two low pass filters forattenuating frequencies above the selected band.

The output from filter 501 is considered to be a first stage signal,present on bus 506. Gain applied to the first stage signal is controlledby the gain control 502 to provide a second stage signal, on bus 507,that is combined with input signal 505 within the summation circuit 504.The original audio input signal is thus processed in combination withthe second stage signal to produce a processed output signal.

The gain control circuit 502 is controlled by the control signal on bus508. This control signal is itself derived from the first stage signal,after receiving a modest degree of processing by circuit 503. The firststage signal is also provided as an audio output on an output line 509.

In use, a gain control circuit 502 provides dynamic control of gainparameters such that, for example, a high degree of gain may be appliedto low level signals with a low degree of gain being applied to highlevel signals; thereby compressing the signal so as to have a smallerdynamic range. However, the treated signal is combined back with theoriginal source, so as to provide a substantially more sophisticatedcompression effect.

Gain control is available so as to adjust the overall degree ofcompression provided. However, the compressed signal is also frequencyselective, such that some frequency components will undergo a greaterdegree of compression than others. Furthermore, the particular rangeover which this compression effect takes place is adjustable.

Experience has shown that the effect is particularly attractive whenapplied to drums, such that the tonal qualities of the drums may bemodified with minimal intervention on the part of the operator, Dynamicmodification of the gain control takes place in real-time in response tofrequency content, such that the modification may take place at a ratethat could not be matched by manual or automated modification to controlparameters directly.

An alternative embodiment is shown in FIG. 6, in which components havingthe same functionality are identified by the same reference numeral asthat used in FIG. 5. In this example, an inverter 601 has been providedbetween the gain control circuit 502 and an input to the summationcircuit 504. In this way, the second stage signal is subtracted from theoriginal audio input signal and not added to the original audio inputsignal. Thus, it is possible to cancel a selected band of frequenciesfrom the main signal. Thus, this may be seen as a dynamiclimiting/gating operation which, in preference to gating the wholesignal (i.e. producing a silence), the gating may be frequencyselective.

An application for such a procedure would be in the control ofsibilance, also commonly know as a “de-esser”. However, it is shouldalso be appreciated that the technique may be deployed in otherenvironments such as when unwanted noise is present at a particularfrequency. Furthermore, having removed the offending frequencies, anoperator may determine the extent to which a proportion of the offendingfrequencies is returned to the original audio signal. Thus, in this wayit may be possible to attenuate an offending signal while at the sametime allowing a proportion of that signal to remain.

Thus, in an outside broadcast for example, it would be possible toattenuate offending frequencies so as to ensure that, for example,commentators and interviewees are heard over background noises while atthe same time ensuring that these noises are still present so as tomaintain a degree of realism. An operator must therefore select anddefine the offending frequencies and then control the extent to whichthose frequencies are attenuated.

In the illustrated examples of FIGS. 5 and 6, an audio input signal isreceived as an original signal that is supplied both to filter 501 andas an input to summation circuit 504 for processing in combination withthe second stage signal as described. Thus, the audio input signal issupplied in the form it is received for processing in combination withthe second stage signal derived from it. Hence, it is to be understoodthat the original audio input signal does not undergo any furtheroperations other than being supplied to the filter prior to beingsupplied for processing in combination with a signal derived from it.

A preferred approach for deploying the functionality of the circuitillustrated in FIG. 6 is detailed in FIG. 7: At step 701, inputs andoutputs are configured for the particular application. Thus, forexample, this may involve receiving several audio inputs from a liveenvironment as part of a radio broadcast. Consequently, a stereo outputis required on line 203 and the signal must be of a broadcast quality.Thus the signal must satisfy the usual requirements for the broadcasterconcerned, while ensuring that the content of the broadcast can be heardclearly but at the same retaining a degree of realism.

At step 702 it is appreciated that the techniques described herein maybe applicable for the application concerned; the techniques have beenidentified as “frequency selective dynamics”. As previously described,the operator is now required to identify the particular frequencycomponents of interest and then use this selection in order to achievean optimal degree of attenuation and then re-mixing a proportion of theoffending component back with the input signal.

Having established this, the system will itself automatically track andattenuate the presence of these components and it is not necessary forthe operator to make any further adjustments. Thus, in this way, ahighly sophisticated degree of processing has been achieved withoutongoing manual intervention on the part of an operator. It should alsobe appreciated that in a broadcast environment, relatively little timeis allowed for experimentation and the operator is therefore underpressure in order to achieve acceptable results in relatively short timescales.

The present procedure facilitates rapid deployment of the frequencyselective dynamics procedure. At step 703 the operator ceases to listento the input signal present at 505 or the output signal produced by thesummation circuit 504. Instead, using headphones 207, the operatoractually listens to the first stage signal produced by filter 501. Whilelistening to this first stage signal, the operator makes adjustments tothe filter frequencies and the operator will aim to maximise the levelof the first stage signal as heard by the headphones 207.

Thus, in order to identify the correct frequency band, the operator isdoing something counter-intuitive in that measures are being taken toincrease the level of the unwanted signal as a procedure for correctlyidentifying its frequency band. Furthermore, an operator will also beaware of the preference for minimising the width of the frequency bandwhile removing substantially all of the unwanted noise. Experiments haveshown that under many operating conditions, operators easily adapt tothis way of working such that undesirable frequency components can beisolated relatively quickly.

After the frequency band of interest has been identified, the operatorthen switches to listening to the actual output signal and by doingthis, a subjective assessment may be made as to the extent to which theoffending noise should be reintroduced, thereby maintaining realism.Thus, it is possible for an operator to achieve a highly sophisticatedresult by just listening to the two signals and making modestadjustments to the controls. By listening to the first stage signalitself (i.e. the offending noise) it is made relatively easy for theoperator to make the appropriate selection and the operator does notneed to rely on sophisticated graphics or other user interfaces.

A second alternative embodiment is illustrated in FIG. 8 and againcomponents providing the same functionality of those shown in FIG. 5have been identified with the same reference numerals. In this example,the original audio input signal is processed in combination with thesecond stage signal and also the first stage signal.

The arrangement of FIG. 8 is similar to that shown in FIG. 5 in that thesecond stage signal at 507 is supplied as an input to summation circuit504. However, in this example, the original audio input signal itselfhas undergone processing before reaching summation circuit 504. For thispurpose, a second summation circuit 801 is provided.

In the circuit of FIG. 8, an audio input signal 505 is received as anoriginal signal that is supplied to summation circuit 801 and to thefilter 501. The first stage signal at 506 is inverted by inverter 802and is then supplied as an input to summation circuit 801. Thus, thefirst stage signal 506 is subtracted from the original audio inputsignal 505 and the resulting output from summation circuit 801, thirdstage signal 803, is supplied as an input to summation circuit 504. Thethird stage signal 803 is then combined with the second stage signal 507by summation circuit 504 to produce a processed audio output signal.

The approach provides for a further level of sophistication in terms ofachieving gating or more preferably compression. The filter 501 selectsa frequency band for compression to be applied. The selected frequencyband is subtracted from the original audio input signal such that theselected band is totally absent from the resulting signal. A proportionof the selected frequency band is then reintroduced by combining thesecond stage signal with the processed input signal. Again, in thecircuit of FIG. 8, the original audio input signal does not undergo anyfurther operations other than being supplied to the filter prior tobeing supplied for processing in combination with a signal derived fromit.

A high level of control is possible given that the frequencies ofinterest are firstly totally removed and then the extent to which areintroduction occurs is controllable by an operator. The use of digitalcircuitry within this environment makes total cancellation possiblegiven that, from any value, its exact opposite is easily calculable. Itis therefore appreciated that techniques of this type may be deployedwithin the digital domain to an extent that would not be achievablewithin a totally analogue environment.

FIG. 9 shows an alternative arrangement to that of FIG. 8 and againcomponents providing the same functionality of those shown in FIG. 5 andFIG. 8 have been identified with the same reference numerals. In thisexample, the original audio input signal is also processed incombination with both the second stage signal and the first stagesignal. However, the individual processing operations of FIG. 9 differfrom those of FIG. 8.

Again, in the circuit of FIG. 9, the original audio input signal doesnot undergo any further operations other than being supplied to thefilter prior to being supplied for processing in combination with asignal derived from it. The arrangement of FIG. 9 is similar to thatshown in FIG. 5 in that the original audio input signal is supplied asan input to summation circuit 504. However, in this example, the secondstage signal itself has undergone processing before reaching summationcircuit 504. For this purpose, a second summation circuit 801 is alsoprovided.

In the circuit of FIG. 9, an audio input signal 505 is received as anoriginal signal that is supplied to summation circuit 504 and to thefilter 501. The first stage signal at 506 is inverted by inverter 802and is then supplied as an input to summation circuit 801. The secondstage signal 507 is also supplied as an input to summation circuit 801.The first stage signal 506 is subtracted from the second stage signal507 and the resulting output from summation circuit 801, third stagesignal 901, is supplied as an input to summation circuit 504. The thirdstage signal 901 is then combined with the original audio input signal505 by summation circuit 504 to produce a processed audio output signal.

Thus, each of the circuits of FIGS. 8 and 9 perform processingoperations to combine the original input signal and the second stagesignal and, additionally, the first stage signal to produce a processedoutput signal. During the processing of the original input signal, thefirst stage signal and the second stage signal, the circuits of FIGS. 8and 9 both utilise a third stage signal. The third stage signal is theresult of a processing operation combining two signals of: the originalinput signal, the first stage signal and the second stage signal. Thethird stage signal is then processed in combination with the remainingof the three signals.

Comparing the circuits of FIGS. 8 and 9 it can be seen that althougheach performs different operations to the other, each receives an audioinput signal, performs an addition of the second stage signal andperforms a subtraction of the first stage signal. Thus, for the sameaudio input signal received at 505, the same first stage signal at 506and the same second stage signal at 507, the circuits of FIGS. 8 and 9produce equivalent output signals.

FIGS. 8 and 9 hence illustrate that in the digital domain differentoperations may be performed to process the audio input signal, thesecond stage signal and the first stage signal in combination thatachieve a common effect.

1. A method of processing an audio input signal to produce a processedaudio output signal, comprising the steps of: receiving the audio inputsignal as an original signal; filtering said audio input signal toproduce a first stage signal of a selected frequency band of said audioinput signal; deriving a control signal from said first stage signal;dynamically controlling gain applied to said first stage signal inresponse to said control signal to produce a second stage signal;combining said first stage signal and said second stage signal toproduce a third stage signal; combining said original signal and saidthird stage signal to produce said processed audio output signal.
 2. Amethod according to claim 1, wherein said step of combining said firststage signal and said second stage signal includes the step ofsubtracting said first stage signal from said second stage signal.
 3. Amethod according to claim 1, wherein said step of combining saidoriginal signal and said third stage signal includes the step of addingsaid original signal with said third stage signal.
 4. (canceled) 5.(canceled)
 6. A method according to claim 1, wherein: the audio inputsignal is one of: a live signal, and a recorded signal; the processedoutput signal is one of: recorded as an audio signal only, recorded incombination with a video signal; and the processed audio output signalis transmitted as one of: a radio broadcast and a television broadcast.7. A method according to claim 1, wherein the audio input signal isdynamically filtered by attenuating frequency components that are belowsaid selected frequency band and attenuating frequency components thatare above said selected frequency band.
 8. (canceled)
 9. (canceled) 10.A method according to claim 1, wherein the level of said derived controlsignal is manually adjustable.
 11. A method according to claim 1,wherein the filtering step and the gain control step are executed bydigital signal processing systems operating in real-time.
 12. Audiosignal processing apparatus for processing an audio input signal toproduce a processed audio output signal, comprising: a filter configuredto pass a selected frequency band of a received input signal; a dynamicgain control configured to control the gain of a signal in response to acontrol signal; and a processor configured to combine signals to producea processed output signal, wherein said apparatus is arranged to receivethe audio input signal as an original signal, said filter is arranged tofilter said audio input signal to produce a first stage signal of aselected frequency band of said audio input signal; said dynamic gaincontrol is arranged to derive a control signal from said first stagesignal to, and control the gain applied to said first stage signal inresponse to said control signal to produce a second stage signal; andsaid processor is arranged to combine said first stage signal and saidsecond stage signal to produce a third stage signal, and combine saidoriginal signal and said third stage signal to produce said processedaudio output signal.
 13. Audio signal processing apparatus according toclaim 12, wherein said processor is arranged to combine said first stagesignal and said second stage signal by subtracting said first stagesignal from said second stage signal, and to combine said originalsignal and said third stage signal by adding said original signal withsaid third stage signal.
 14. Audio signal processing apparatus accordingto claim 12, wherein said filter includes a low pass component and ahigh pass component in order to pass said selected frequency band. 15.Audio signal processing apparatus according to claim 12, wherein saidfilter and said dynamic gain control are constructed from digital signalprocessing systems operating in real-time.
 16. A non-transitorycomputer-readable medium having computer-readable instructionsexecutable by a computer such that, when executing said instructions, acomputer will perform the steps of: receiving an audio input signal asan original signal; filtering said audio input signal to produce a firststage signal of a selected frequency band of said audio input signal;deriving a control signal from said first stage signal; dynamicallycontrolling gain applied to said first stage signal in response to saidcontrol signal to produce a second stage signal; combining said firststage signal and said second stage signal to produce a third stagesignal; combining said original signal and said third stage signal toproduce a processed audio output signal.
 17. (canceled)
 18. (canceled)19. (canceled)
 20. A non-transitory computer-readable medium accordingto claim 16, wherein a computer executing said instructions isconfigured such that the level of said derived control signal ismanually adjustable.
 21. A non-transitory computer-readable mediumaccording to claim 16, wherein when executing said instructions acomputer will combine said first stage signal and said second stagesignal by subtracting said first stage signal from said second stagesignal.
 22. A computer-readable medium according to claim 16, whereinwhen executing said instructions a computer will combine said originalsignal and said third stage signal by adding said original signal withsaid third stage signal.